1. Field of the Invention
The present invention relates to an apparatus for detecting combustion state of or quality an internal combustion engine on the basis of a change in an ion current which is generated upon combustion of an air-fuel mixture in the engine. More particularly, the invention is concerned with a combustion state detecting apparatus for an internal combustion engine which is provided with a means for preventing a bias voltage applied to an electrode of a spark plug for detecting the ion current from lowering, to thereby ensure high reliability for the detection of ion current and hence the combustion state or quality of the engine.
2. Description of Related Art
In general, in the internal combustion engine including a plurality of cylinders, an air-fuel mixture is charged into a combustion chamber defined within each of the engine cylinders to be subsequently compressed during a compression stroke by a piston moving reciprocatively within the cylinder. Subsequently, a high voltage is applied to a spark plug of the cylinder, whereby a spark is generated between electrodes of the spark plug due to electric discharge. Thus, combustion of the compressed air-fuel mixture is triggered. Explosion energy resulting from the combustion is then converted into a movement of the piston in the direction reverse to that of the compression stroke, which motion is translated into a torque outputted from the internal combustion engine via a crank shaft.
Upon combustion of the compressed air-fuel mixture within the engine cylinder, molecules prevailing within the combustion chamber are ionized. Thus, by applying a bias voltage to an ion current detecting electrode (which is usually constituted by an electrode of the spark plug and which is mounted as exposed to the interior of the combustion chamber, an amount of ions carrying electric charges flows between the electrodes of the spark plug. Thus, an ion current is generated.
As is known in the art, magnitude of the ion current varies with a high sensitively in dependence on the combustion state or quality within the combustion chamber. By taking advantage of this phenomenon, the combustion state within the engine cylinder can discriminatively be identified or determined by detecting behaviors or attributes of the ion current such as a peak value thereof and the like.
The combustion state detecting apparatus for the internal combustion engine of the type mentioned above is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 104978/1990 (JP-A-2-104978). More specifically, there is disclosed in this publication an apparatus for detecting such abnormality of the engine operation as typified by occurrence of the misfire on the basis of the ion current detected immediately after the combustion by using the electrodes of the spark plug as the electrodes for detecting the ion current.
The bias voltage for detecting the ion current has to be applied to the spark plug with a same polarity as a high firing voltage (i.e., voltage for firing the air-fuel mixture) via a diode capable of withstanding a high voltage. In this conjunction, it is however noted that the spark plug is connected to the output terminals of the secondary winding of an ignition coil for which is employed applying the high voltage to the spark plug.
As is apparent to those skilled in the art, upon starting of a current supply to a primary winding (i.e., start of energization of the primary winding) of the ignition coil, a voltage is induced in the secondary winding with a polarity reverse to that of the secondary voltage induced upon interruption of the primary current. Consequently, there may arise such an unwanted situation that the bias voltage is lowered because a discharge current will then flow to the ignition coil.
Such lowering of the bias voltage may equally take place even when a high-voltage distribution system is adopted in which a firing high-voltage is applied to the spark plug via a distributor, because of possibility of discharge from the peripheral electrodes to the center electrode of the distributor.
As will now be apparent from the above, the conventional combustion state detecting apparatus for the internal combustion engine known heretofore suffers a problem that when the ion current detection unit is connected to the secondary winding of the ignition coil the peripheral electrodes of the distributor, a voltage having a polarity reverse to that of the firing voltage is generated upon starting of the electric energization of the ignition coil. Thus, the bias voltage can not be prevented from lowering due to discharge of the bias voltage to the ignition coil, which is of course disadvantageous in that sensitivity and reliability for the ion current detection and hence for the combustion state determination are thereby degraded.